In recent times, as portable electronic appliances, such as notebook computers, video cameras and cellular phones, have been in increasingly in demand and development of electric vehicles, energy storage capacitors, robots and satellites have been regularized, active studies have been conducted about high-performance secondary batteries capable of repeating charging/discharging.
Recently, commercially available secondary batteries include nickel cadmium batteries, nickel metal hydride batteries, nickel zinc batteries and lithium secondary batteries. Among those, lithium secondary batteries cause little memory effect as compared to nickel-based secondary batteries to allow free charging/discharging, have a significantly low self-discharge ratio and high energy density, and thus have been given many attentions. In general, the secondary batteries may be classified into can-type secondary batteries and pouch-type secondary batteries, depending on the type of a casing or application.
FIG. 1 and FIG. 2a are exploded perspective views illustrating the structure of the conventional pouch-type secondary battery. As shown in FIG. 1, the conventional pouch-type secondary battery 1a includes an electrode assembly 20 having electrode terminals 21 and a pouch casing member 10a configured to receive the electrode assembly 20.
Referring to FIG. 1, sealing portions 40 are provided at the circumference of the electrode receiving portion and the total length La of the battery is determined by the length of the electrode assembly receiving portion and the width of the two sealing portions 40 provided at both ends of the electrode assembly receiving portion. Therefore, when an upper pouch member 11a and a lower pouch member 12a are provided individually as shown in FIG. 1, sealing portions are formed at all of the four sides of the electrode assembly receiving portion, and thus the total length is increased by the width of the sealing portions.
FIG. 2a is an exploded perspective view of another type of conventional pouch-type secondary battery 1b, wherein an upper pouch member and a lower pouch member are formed integrally in one pouch film. Referring to FIG. 2a, an electrode assembly receiving portion is formed in one pouch member, a predetermined portion of the pouch film is folded, the opening of the electrode assembly receiving portion is covered with the other pouch member, and then the upper pouch member and the lower pouch member are sealed. In this case, sealing portions are formed at the three lateral side portions, except the connection between the upper pouch and the lower pouch. The secondary battery 1b has a total length La decreased by the width of one sealing portion, as compared to the secondary battery 1a. However, in the case of the connection having no sealing portion, a part thereof protrudes toward the outside undesirably, after sealing. FIG. 2b is a photographic image illustrating an actual embodiment of the secondary battery type 1b, wherein a folding portion where the upper pouch member is connected with the lower pouch member protrudes. As a result, even when forming the pouch casing material in the type of secondary battery 1b, there is no significant effect of reducing the total length as compared to the secondary battery 1a. It is thought that this phenomenon is caused since the regions of the upper pouch member and the lower pouch member around the connected portion come into contact with each other due to the vacuum applied to prevent air from remaining in a battery upon sealing, after injecting an electrolyte into a pouch casing material.
Therefore, there is a need for developing a pouch casing material configured to prevent protrusion of a connection between an upper pouch member and a lower pouch member after sealing.